In the conventional internal combustion engines, gas exchange takes place via spring-loaded valves that are opened by a camshaft. The sequential operation of the valve lift, that is, the beginning and the duration of the valve opening or position of the valve tappet are determined by the shaping of the camshaft, but are generally not variable. In order to improve the efficiency of internal combustion engines, and also with regard to reducing exhaust gas, use is increasingly being made of concepts for the variable activation of gas exchange valves. For example, by changing the phase position of the camshaft, the intake and exhaust times of gas exchange valves are able to be varied. A flexible operation of an internal combustion engine is possible if the gas exchange valves are not activated by a camshaft, but are activated directly.
In engines having variable valve timing, in order to control the gas exchange, the gas exchange valves are opened and closed using electrohydraulic or electromagnetic control elements, instead of being conventionally activated via camshafts. Valve lift and valve control times are thereby able to be selected to be variable. The control of these control elements takes place as a function of various operating parameters of the engine, by software-controlled electronic control units.
In this context, appropriate actuator control signals are generated from the gas exchange valve control specifications, such as lift and control times, and taking into consideration the system state variables, such as supply voltage, combustion chamber pressure, pressure characteristics in the rail, temperatures, oil properties, etc.
In variable valve control, in contrast to a classical camshaft activation, since mechanical positive coupling is missing between camshafts and gas exchange valve, there is a danger that, in a faulty activation, a gas exchange valve collides with another gas exchange valve (a so-called “valve-valve collision,” also referred to hereinafter by the acronym VVK) or with a piston of the internal combustion engine (a so-called “valve-piston collision,” also referred to hereinafter by the acronym VKK). Such collisions typically lead to damage of the components, possibly including destruction of the internal combustion engine.
Thus, for instance, too great a valve lift at a point in time or at a crankshaft angle, e.g., at the time when a piston sweeps over its upper dead center OT, may have the effect that the gas exchange valve collides with the piston. Control specifications that lead to collision may be caused, for instance, by typing errors during the programming of the control specification.
For example, published German patent document DE 198 57 193 describes a method for testing the operativeness of a variable valve control, in which the combustion chamber charge is determined via the valve control. If the combustion chamber charge that has been tested by air mass measurements deviates from a setpoint value, it is concluded that there is an error in the valve control, and an error signal is generated.
Smaller deviations and errors in the valve control may thus be detected without further ado and may possibly be remedied. However, in order reliably to prevent collisions of the gas exchange valves, it is necessary in the operation of internal combustion engines having variable valve control to take further precautions.